CONCRETE FILLED HSS - Steel Tube Institute

CONCRETE FILLED HSS

KIM OLSON, PE

HSS

Steel Tube Institute Hollow Structural Sections ® 2020 Steel Tube Institute. All Rights Reserved

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• Why CFT?

• Section Classifications

• Analysis Methods

• Design Requirements

• Recommendations

• Practical Considerations

Agenda

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Composite Members

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§Two types§Encased composite members§ Filled composite members

OptionalReinforcement


Concrete Filled HSS

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§Two components§Steel§Concrete

§ Unreinforced or reinforced§ NWC or LWC§ High-strength, steel fiber reinforced, self-

consolidating mix§Members under static, impact, blast, seismic and fire

loads§Gain benefits from the composite action of the two

materials


Advantages of CFT

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§Elimination of need for formwork§ Increased member capacity, stiffness, ductility§ Increased fire resistance

§May negate need for external fire protection§May increase connection strength§Reduction of HSS size§Protection for concrete


Disadvantages of CFT

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§Need to account for concrete shrinkage in HSS§Lack of knowledge of mechanical bond between

concrete and steel§Coordination between trades

zSECTIONCLASSIFICATION


Materials

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§Steel§A500, A1085§ Fy ≤ 75 ksi

§Concrete§NWC 3 ksi ≤ f’c ≤ 10 ksi§ LWC 3 ksi ≤ f’c ≤ 6 ksi

§Reinforcing§A615 Gr. 40, 60, 75, 80§ Fy ≤ 80 ksi


• Classify section as compact, non-compact or

slender based on lowest element slenderness

ratio for HSS

• Concrete infill changes the buckling mode

within the cross-section and along the length of

the member

• Influences strength, stiffness and ductility

Section Classification

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Hollow Concrete Filled


• b = clear distance between webs less than

inside corner radius on each side

• b = B – 3tdes

Section Classification

12


Slenderness Limits – Axial Compression

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ElementWidth-

ThicknessRatio

𝝀𝒑Compact/

Noncompact

𝝀𝒓Noncompact/

SlenderMax

Permitted

Concrete Filled Rect. HSS

𝑏𝑡

2.26𝐸𝐹#

3.00𝐸𝐹#

5.00𝐸𝐹#

Hollow Rect. HSS

𝑏𝑡

1.40𝐸𝐹#

Concrete Filled Round HSS

𝐷𝑡

0.15𝐸𝐹#

0.19𝐸𝐹#

0.31𝐸𝐹#

Hollow Round HSS

𝐷𝑡

0.11𝐸𝐹#

Filled HSS9x9x1/8 is slender

ALL filled square HSS compact except: HSS7x7x1/8, HSS8x8x1/8, HSS10x10x3/16, HSS12x12x3/16

All filled round HSS are compact


Slenderness Limits – Flexure

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ElementWidth-

ThicknessRatio

𝝀𝒑Compact/

Noncompact

𝝀𝒓Noncompact/

SlenderMax

Permitted

Concrete Filled Rect. HSS Flange

𝑏𝑡

2.26𝐸𝐹#

3.00𝐸𝐹#

5.00𝐸𝐹#

Hollow Rect. HSS Flange

𝑏𝑡

1.12𝐸𝐹#

1.40𝐸𝐹#

Concrete Filled Rect. HSS Web

ℎ𝑡

3.00𝐸𝐹#

5.70𝐸𝐹#

5.70𝐸𝐹#

Hollow Rect. HSS Web

ℎ𝑡

2.42𝐸𝐹#

5.70𝐸𝐹#

Concrete Filled Round HSS

𝐷𝑡

0.09𝐸𝐹#

0.31𝐸𝐹#

0.31𝐸𝐹#

Hollow Round HSS

𝐷𝑡

0.07𝐸𝐹#

0.31𝐸𝐹#

Filled HSS9x9x1/8 is slender

All filled square HSS compact except: HSS7x7x1/8, HSS8x8x1/8, HSS10x10x3/16, HSS12x12x3/16

No filled round HSS is slender

All filled round HSS compact except: HSS6.625x0.125, HSS7.000x0.125, HSS10.000x0.188, HSS14.000x0.250, HSS16.000x0.250, HSS20.000x0.375

zANALYSIS METHODS


Four Methods – AISC 360-16 I1.21 Plastic Stress Distribution

2 Strain Compatibility

3 Elastic Stress Distribution

4 Effective Stress-Strain

Nominal Strength

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Plastic Stress Distribution Method

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§ Based on plastic limit analysis (form plastic hinge)§ Assume steel components have reached Fy (tension or

compression)§ Compression in concrete due to axial or flexure reaches a stress

of§ 0.85f’c (rectangular)§ 0.95f’c (round)

§ Assumptions:§ Sufficient strains to reach yield strength (concrete & steel)§ Local buckling delayed until after yielding or concrete

crushingMethod used for AISC’s Composite Design Tables


Strain Compatibility Method

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§More general approach§ Assume linear distribution of strain across section§ Max concrete compressive strain = 0.003 in./in.§ Stress-strain relationship from tests or published results

Works for noncompact and slender sections


Elastic Stress Distribution Method

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§Retained from previous specifications to allow for composite beams with noncompact webs

§Determine nominal strength from superposition of elastic stresses§ Limit state of yielding§Concrete crushing

§Useful when plastic stress distribution is not applicable§ In practical terms, does not apply to HSS


Elastic Stress - Strain Method

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§Alternative approach for noncompact and slender sections§Assume strain compatibility§Applicable when using a fiber-based approach for axial force-moment

strength interaction§Sophisticated computer analysis – not intended for typical design

practice

zDESIGN REQUIREMENTS


Required Strengths

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Account for stability• Amplified first-order analysis (App 7)• Second-order analysis (Ch C)• Direct analysis• Use effective flexural stiffness of composite section• Nominal axials stiffness is sum of elastic axial

stiffness of HSS and concrete• Apply stiffness reduction factor (0.64 for flexural

stiffness and 0.8 for nominal axial stiffness)


Effective Stiffness

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𝐸𝐼!"" = 𝐸#𝐼# + 𝐸#𝐼#$ + 𝐶%𝐸&𝐼&

Effective rigidity of filled HSS

𝐶% = 0.45 + 3𝐴# + 𝐴#$𝐴'

≤ 0.9

Rectangular HSS considering HSS corners

𝐼&( =𝐵 − 4𝑡 ℎ)%

12+𝑡 𝐻 − 4𝑡 %

6+

9𝜋* − 64 𝑡+

36𝜋+ 𝜋𝑡*

𝐻 − 4𝑡2

+4𝑡3𝜋

*

𝐼&, =𝐻 − 4𝑡 𝑏)%

12+𝑡 𝐵 − 4𝑡 %

6+

9𝜋* − 64 𝑡+

36𝜋+ 𝜋𝑡*

𝐵 − 4𝑡2

+4𝑡3𝜋

*


Nominal Strength – Tension

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Limit State of Yielding§Only steel components (HSS and reinforcing) contribute

to the tensile strength§𝑃- = 𝐴#𝐹, + 𝐴#$𝐹,#$

HSS reinforcing §𝜙. = 0.90 (LRFD)§Ω. = 1.67 (ASD)

Use AISC Manual Table 5-4


Nominal Strength – Compression

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§Based on a summation of the strengths of all components (HSS, concrete and reinforcing)

§Function of member slenderness (compact, noncompact, slender)§Reduce strength due to member slenderness§Reduce strength due to local buckling§ Thinner HSS cannot adequately confine concrete

infill at stresses above 0.7f’c.


Nominal Strength – Compression

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§Limitations§Steel must comprise at least 1% of total cross-

section§Minimum longitudinal reinforcing is not required

§General procedure§Calculate zero-length strength accounting for the

influence of local slenderness§Consider length effects and end conditions (limit

state of flexural buckling)


Compressive Strength – Compact Section

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Develop yielding in HSS and compressive strength of the concrete fill

𝑃-/ = 𝑃0

𝑃0 = 𝐴#𝐹, + 𝐶*𝑓&1 𝐴& + 𝐴#$2!2"

material transformation𝐸! = 𝑤!".$ 𝑓!%

Zero length nominal axial strength

Full plastic strength

0.85 (RHS)0.95 (CHS)


Area of Concrete - RHS

Simplified

𝐴& = 𝐵 − 2𝑡 𝐻 − 2𝑡 = 𝑏)ℎ)

𝐴& = 99 𝑖𝑛*

𝐴& = 28.89 𝑖𝑛*

Considering Corners

𝐴& = 𝑏)ℎ) − 𝑡* 4 − 𝜋

𝐴& = 98.79 𝑖𝑛*

𝐴& = 28.56 𝑖𝑛*

HSS12x10x1/2

HSS6x6x5/8


Compressive Strength – Noncompact Section

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Develop yielding in HSS but not confine the concrete fill after it reaches a compressive stress of 0.7fc’

𝑃-/ = 𝑃0 −𝑃0 − 𝑃,𝜆$ − 𝜆0

* 𝜆 − 𝜆0*

𝑃, = 𝐴#𝐹, + 0.7𝑓&1 𝐴& + 𝐴#$2!2"


Compressive Strength – Slender Section

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Cannot reach yielding in HSS (limited to critical buckling stress, Fcr) and cannot confine the concrete fill after it reaches a compressive stress of 0.7fc’

𝑃-/ = 𝐴#𝐹&$ + 0.7𝑓&1 𝐴& + 𝐴#$𝐸#𝐸&

𝐹&$ =72!#$

% (RHS) 𝐹&$ =8.:*;&'$(&)!

% (CHS)


Zero Length Nominal Compressive Strength

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𝜆0 𝜆$ Max

𝑃! = 𝐹"𝐴# + 𝐶$𝑓%& 𝐴% + 𝐴#'𝐸#𝐸%

𝑃() = 𝑃! −𝑃! − 𝑃"𝜆' − 𝜆!

$ 𝜆 − 𝜆!$

𝑃() = 𝐹"𝐴# + 0.7𝑓%& 𝐴% + 𝐴#'𝐸#𝐸%

HSS Element Slenderness, l

Nom

inal

Sec

tion

Stre

ngth

, Pno


Design Compressive Strength

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𝑃-/ = 𝐹,𝐴# + 𝐹,#$𝐴#$ + 0.85𝑓&1𝐴&

𝑃! =<% 2=*++

>"%

𝑃-/𝑃!

≤ 2.25𝑃-/𝑃!

< 2.25

𝑃- = 𝑃-/ 0.658?,-?*

𝑃- = 0.877𝑃!


Compression of Bare Steel

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§Composite and Non-Composite HSS have different SF§Composite (fc = 0.75, Wc = 2.00)§Non-Composite (fc = 0.9, Wc = 1.67)

§Difference can result in the no—composite section having a higher strength

§Take the larger of the two values


Nominal Flexural Strength

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§Function of member slenderness§HSS section reduces lateral-torsional instability§Concrete infill changes local buckling mode§General procedure:

§Classify the section§Calculate nominal strength§Neglect variation of stress through the thickness of the flange§Equations provided based on plastic stress distribution method

for compact sections§Use strain compatibility for noncompact and slender concrete

filled HSS


Nominal Shear Strength

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§ Three options§Shear strength based on HSS alone (Chapter G)§Shear strength based on reinforced concrete

alone (ACI 318)§Sum of shear strength of HSS and the reinforcing

steel

zPOLL QUESTION


Load Transfer

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§How much of applied load is resisted by HSS? How much resisted by concrete?§External force applied to HSS only§External force applied to the concrete only§External force applied to both

§Assumes load transfer occurs between materials to achieve equilibrium across the cross-section


Load Transfer

38

HSS to Concrete (external force applied to HSS)

𝑉$1 = 𝑃$ 1 − ;&@!?,-

(I6-1)

Concrete to HSS (external force applied to concrete only)

𝑉$1 = 𝑃$;&@!?,-

(compact & non-compact) (I6-2a)

𝑉$1 = 𝑃$;".@!?,-

(slender) (I6-2b)

Concurrent application

𝑉$1 = 𝑃$,&/-&$!.! − 𝑃$ 1 −𝐹,𝐴#𝑃-/

𝑉$1 = 𝑃$,BCC − 𝑃$𝐹,𝐴#𝑃-/


Load Transfer

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Cannot superimpose mechanisms – use largest• Direct bearing (bearing pl) 𝑅- = 1.7𝑓&1𝐴D (I6-3)• Shear connection (hdas, channel anchors) 𝑅- = ∑𝑄&E (I6-4)• Direct bond 𝑅- = 𝑝F𝐿)-𝐹)- (I6-5)

B or d

Load transfer region

2B or 2d

Max load introduction length

= ⁄12𝑡 𝐻& for rectangular HSS= ⁄30𝑡 𝐷& for rectangular HSS

zDesign Recommendations


Design Recommendations

§ Limit to compact HSS member

§ Use C-I3.7 equations for noncompact and slender RHS

§ Use plastic stress distribution method for beam-columns§ Use P-M equations from EJ Discussion

(Geschwinder, 2010)

§ Choose f’c = 4 ksi to 5 ksi – benefits above 5 ksi are

limited



Concrete Strength Effect: Member Sizef’c = 4 ksi versus 6 ksi



Concrete Strength Effect: Member Thicknessf’c = 4 ksi versus 6 ksi

zPractical Considerations


Practical Considerations

CIDECT Design Guide #5

Jeffrey A. Packer


§ Concrete mix§ High strength grout can also be

used to fill HSS in lieu of concrete§ Care must be taken when filling the

HSS from top§ Segregation of concrete§ Damage to HSS walls

§ Consider constructability of the column is rebar is used§ Interference with the connection§ Adequate rebar spacing to place

the concrete


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§Concrete placement§ Typically done after erection§Pumped through an opening in

the top of bottom of the column using the same types of equipment that would be required for filling tall concrete wall forms

Concrete Placement

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Jeffrey A. Packer


• Alternate concrete placement• Using a staging area where the

columns are filled in a vertical or inclined position• All columns could be filled at

one time

Concrete Placement

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CIDECT Design Guide #5


Concrete Placement

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§Ensure concrete doesn’t segregate during placement per ACI 318§Avoid any situation where concrete could free fall

and hit the HSS walls, reinforcing, etc. within the HSS cavity

§Rule of thumb – concrete should not free fall more than 3 to 5 feet

§See also ACI 304R, Chapter 5


Concrete Placement

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§Concrete placed at top§Use a tremie chute§Concrete may need to be vibrated to achieve

adequate consolidation§Some contractors prefer a self-consolidating

concrete mix to avoid the concrete vibration§Detail access at the top of the column


Concrete Placement

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§Concrete placed at top - access§Simple where no cap plate is required§Cap plate required§Specify field welding after concrete placement or§Detail a hole to allow for concrete placement


Recommendation

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§Specify holes in the column§Approximately 1” in diameter§Venting and concrete fill verification§Vent steam during an active fire scenario§Pressure due to buildup of steam within the HSS

can deform or rupture the HSS


Concrete Placement – Tall Column

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§Pump concrete in lifts through openings spaced along the length of a column§Similar to procedure for a tall wall form§See ACI 304R§Holes required are fairly large – their impact on the

column capacity must be evaluated



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§Connection Geometry§ Through plate and diaphragm plate connections are

problematic



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§Connection Geometry§Cut out plate or direct

connections remove interior plates



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§Bearing at the top of the column§Detail the top of column condition to

ensure load is transferred to the column in a manner that is consistent with the design assumptions

§Concrete shrinkage results in a void between the top of column and underside of column cap pl§Provide 2-4” of NS grout at the top§Provide embedded anchorage at the

underside of the cap pl


Concrete used as Fire Protection

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§During a fire event, the full axial load is carried by the concrete alone§Design of the column and connecting members must

consider the load path§More information on Fire Protection of HSS on STI

website

zPOLL QUESTION


z

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THANK YOU

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CONCRETE FILLED HSS - Steel Tube Institute

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